Cleanup the externals tests by introducing a couple of helper functions to  check existence of working copy paths.
Cleanup the externals tests by introducing a couple of helper functions to 
check existence of working copy paths.

* subversion/tests/cmdline/externals_tests.py:
  (probe_paths_exist): New helper which checks the existence of a list of paths.
  (probe_paths_missing): New helper which checks for the lack of existence of
  a list of paths.
  (checkout_with_externals, update_receive_new_external, update_lose_external,
   update_change_pristine_external, update_change_modified_external,
   modify_and_update_receive_new_external, export_with_externals):
  Update to use the new helper functions.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@863511 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion